1. Why modify gravity? Explain Tier I problems: Extra gravity in galaxies & clusters Today In growth of baryonic structure post-recombination Accelerating expansion

2. Why modify gravity? Explain: Tier I problems: (1) extra gravity; (2) acceleration Tier II problems/mysteries/coincidences: Absence of large angle correlations in CMB (C(θ>θ c ≈60 o )≈0) Pioneer anomaly Coincidence of scales: (v 2 /r) galaxies ≈ H o c ≈ (H o c)/θ c ≈ Δa Pioneer Tier III vague musings: “inflationary” problems

3. Absence of large angle correlations WMAP1: S 1/2 =  -1 1/2 C(  ) 2 dcos  C. Copi, D. Huterer, D.Schwarz & G.Starkman

4. Dark matter Dark energy modified gravity (Inflaton) simple, unified, predictive, testable

5. gravitational “detection of dark matter”

6. Growth of structure Any theory that respects Birkhoff’s Law, does not result in the observed growth of structure (Lue, Scoccimaro, Starkman Phys.Rev.D69:044005,2004. e-Print: astro-ph/0307034).

7. Solution: Break Birkhoff’s law. How: Allow new fields to seed growth of structure Modify Poisson equation (MOND) Eg. TeVeS ( Bekenstein astro-ph/0403694 ), GEA ( Zlosnik, Ferreira, GDS, astro-ph/0607411 )

8. Good news: New gravitationally coupled fields that seed structure can cause weak lensing; could separate from baryons  “Bullet cluster” (Dai, Matsuo, Starkman, Phys.Rev.D78:104004,2008. arXiv:0806.4319)

9. Problem: Without Birkhoff’s law, the field here depends on the distribution of matter everywhere in some complicated way On interior of a spherical shell

10. Good news: MOND has a Birkhoff’s law Bad news: On interior of a spherical shell This doesn’t help It is the 1/r 2 of Newtonian gravity that counts

12. Summary Modified gravity theories could reproduce the successes of DM/DE, but at the expense of becoming like DM/DE theories -- wysiNwyg and with the possible loss of reliable calculability.